If you give in to the LED, they will start asking for an on/off switch.

Best not to get started down that road though, otherwise the next thing you know, there's a fancy fluorescent display and a microcontroller on board, which really can mess with the audio signal (and the budget).

I actually wonder how John copes with that sort of thing with his commercial design work. As I understand it, he designs the core audio circuits, but the peripheral (or supporting) circuitry is left to others. I'd expect there are cases where the peripheral stuff hurts the performance of the core circuit.

Yes, quite a few people have. Note that I use LEDs effectively in series with the signal in my MC preamp and the signal to noise performance of the preamp is close to the theoretical maximum. Typical red LEDs are 0.4nV/rt Hz with noise corners below 100 Hz.

Leaving indicators off a preamp is performance art, not performance.

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"Pity, wrath, heroism, filled them, but the power of putting two and two together was annihilated."- EM Forster

Red LEDs are not too terrible as photodiodes and I've played around with them for optocoupling where I wanted a big separation of emitter and detector. I tested shorter-wavelength parts and found very low photoelectric sensitivity. However I'm not sure if the latter are as good a tempco match to silicon Vbe.

I recently made a low battery detector using an LED-biased I source as the reference. The accuracy requirements are minimal. But I was pleased to get useful functionality with only 4uA of drain at 9V. This is for an apartment security system and one of the requirements was building it entirely out of parts I already had around

Too far afield to recapitulate, but somewhere in one of the threads I told the story of using an LED to bias a common-base stage for an automotive amp, and the disasterous results of manufacturing's forcing the leads into the wrong holes on the PCB before wavesoldering.

If you give in to the LED, they will start asking for an on/off switch.

Best not to get started down that road though, otherwise the next thing you know, there's a fancy fluorescent display and a microcontroller on board, which really can mess with the audio signal (and the budget).

I actually wonder how John copes with that sort of thing with his commercial design work. As I understand it, he designs the core audio circuits, but the peripheral (or supporting) circuitry is left to others. I'd expect there are cases where the peripheral stuff hurts the performance of the core circuit.

This is a very serious problem of course. At least LCDs are low power, and there's e-ink now that is even static for a steady display. And don't even get started on wireless this and wireless that.

Nelson may remember or be alluding above to one of the British mags from many years ago (was it What HiFi? ?) that had someone saying "You're not going to like this", and then proceeding to describe how the sound of a component changed when he disabled the power indicator LED. A bit far-fetched, but then one could imagine a way in which the change in current could have a tiny effect on a poorly-designed circuit.